Health of the ocular surface is dependent on tear fluid secretions from the lacrimal gland. The lacrimal acinar cells comprising the lacrimal gland are polarized and highly differentiated tear secreting cells that adhere to a complex periacinar basement membrane. The bulk of the apical cell cytoplasm contains large secretory granules packed with tear proteins. Known tear proteins include: lysozyme, which plays a prominent bacteriocidal role on the corneal surface; lactoferrin, which functions as both a bacteriocidal agent and as a potential inhibitor of complement activation; secretory component, which regulates the transcellular movement of IgA into acini lumen where it acts on the corneal surface to inhibit bacterial adhesion; and tear lipocalins (tear-specific prealbumin) and growth factors TGFα, TGFβ and EGF the functions of which are not known. In rats, peroxidase is a tear component which has served as a convenient marker in experimental studies. Tears not only have an important bacteriocidal role, they also keep the cornea clean and lubricated and are important for the well-being of the corneal epithelium.
When lacrimal acinar cell tear output is collectively deficient, ‘Dry Eye’ (also known as keratoconjunctivitis sicca [KCS]); is the result. Dry Eye is a common ocular manifestation of Sjogren's syndrome, an autoimmune disease with unknown etiology that affects millions of people worldwide. Most commonly affected are post-menopausal women with varying degrees of severity. If untreated, Dry Eye can lead to corneal abrasion, ulceration, bacterial infection and loss of vision. Molecular mechanisms underlying the pathogenic decline of secretory output by the main lacrimal gland are potentially multiple. Lacrimal glands of Sjogren's syndrome patients contain foci of B and T lymphocytes whose pathogenic expansion, possibly due to viral insult, can destroy lacrimal acini. However, acinar volume loss often appears insufficient relative to the theoretical overcapacity of the main lacrimal gland. Estimates suggest a potential secretory output up to ten-fold greater than is required to maintain a normal aqueous tear film layer. Other mechanisms therefore warrant attention, such as aberrant secretion of one or several common cytokines that may directly or indirectly alter lacrimal acinar cell function and/or lead to a decline in neural innervation. Novel autocrine/paracrine factor(s) released by lacrimal acinar cells into the tear film may be required for the health of the lacrimal secretory machinery, ductal system and corneal epithelium. The periacinar basement membrane is also required for normal secretory function, in part via ‘BM180’ whose apparent synergy with laminin-1 promotes stimulated tear secretion. Alteration of each of these factors, together or independent of hormonal changes, could contribute to decreased secretory capacity.
Existing protocols for treating Dry Eye suffer from several limitations. In particular, topical artificial tear replacement fluids are widely distributed by a number of pharmaceutical companies, but the efficacy is poor and short-lived. This lack of efficacy is due in part to the fact that constituents of natural human tears are only partially known.
The present invention is directed to a novel human extracellular glycoprotein termed ‘lacritin’ that is remarkably reduced in Sjogren's syndrome. Furthermore lacritin has been found to act in an autocrine manner to enhance unstimulated (but not stimulated) tear secretion. Lacritin is produced by lacrimal acinar cells and released for the most part into tear fluid—much like acinar cell-expressed TGFβ's. This glycoprotein acts like a growth factor when added in purified recombinant form to cultures of human corneal epithelial cells, and in a feedback mechanism, it also appears to act on the same lacrimal gland cells that produce it. Accordingly in one embodiment of the present invention, lacritin is included as an active agent in artificial tear products.